Small cleanups and optmizations. 6% speedup on D8.

Bug:
Change-Id: I11540ed24dc1e03a245bf31b06b5754bfccc2e55
diff --git a/src/main/java/com/android/tools/r8/graph/Code.java b/src/main/java/com/android/tools/r8/graph/Code.java
index 54f9470..3845507 100644
--- a/src/main/java/com/android/tools/r8/graph/Code.java
+++ b/src/main/java/com/android/tools/r8/graph/Code.java
@@ -11,7 +11,7 @@
 import com.android.tools.r8.naming.ClassNameMapper;
 import com.android.tools.r8.utils.InternalOptions;
 
-public abstract class Code extends CanonicalizedDexItem {
+public abstract class Code extends CachedHashValueDexItem {
 
   public abstract IRCode buildIR(DexEncodedMethod encodedMethod, InternalOptions options);